The ever more difficult search for petroleum has led to exploration in areas which were thought by many just a short time ago to be incapable of producing petroleum at an economically feasible price. The rising price of petroleum coupled with its relative domestic scarcity has made acceptable the costs associated with production in Alaska and the North Sea, as well as in a number of offshore areas.
Of the many methods used in prospecting for subsea petroleum, few have attained as widespread an acceptance as has the use of towable marine seismic sources.
The theory of operation in using marine seismic sources to search for petroleum is quite simple. A seismic signal is introduced into the water body. The wave propagates down through the water, across the water-floor interface, and into subfloor geologic formations. The resultant echoes are, to some extent, reflected back across the same path to an array of hydrophones located near the water's surface. Analysis of the signals produced by the hydrophones can provide some information concerning the structure of the subfloor geological formations and attendant petroleum accumulation in those formations.
The term "water" as used herein is meant to include swampwater, mud, marshwater, seawater or any other liquid containing sufficient water to enable operation of the invention.
There are many different methods of producing a seismic pulse. For instance, the earliest attempts entailed the use of solid explosives. This method produces a strong low frequency signal which, accordingly, achieves substantial penetration into subfloor geologic formations and a strong return echo. However, solid explosives possess certain inherent drawbacks: they are dangerous to store, handle, and use. When set off in open water, they kill marine life. In a crowded area such as a harbor, they cannot be used at all. Solid explosives are orders of magnitude more expensive to use, on a per-shot basis, than are most other acoustic sources. Modification of the signature to achieve an acceptable frequency spectrum distribution is most difficult.
Apparatus using explosive gas mixtures, e.g., propane and oxygen, to produce a seismic signal in the form of a pulse or shock wave have gained wide acceptance. The two major types of explosive gas guns are: first, those which operate by exploding a gas mixture behind a flexible membrane which is in contact with the water; and, second, those which operate by allowing the abrupt bubble from the gas explosion to pass directly into the water. An example of the former apparatus can be found in U.S. Pat. No. 3,658,149; an example of the latter apparatus can be found in U.S. Pat. No. 4,193,472.
Other devices using high pressure compressed gases to generate a seismic pulse have also gained wide acceptance in the industry. These apparatus, or guns, typically employ a gas-holding chamber which is pressurized to attain some pre-set level and is fired by allowing the pressurized gas to explosively exit the gun into the surrounding water. Examples of open-ported pressurized gas guns are found in U.S. Pat. No. 3,653,460, to Chelminski, and U.S. Pat. No. 4,141,431, to Baird.
The device of the present invention is a member of a class which generates a relatively instantaneous low-power and low-frequency (10-100 Hz) signal known as a "chirp" which extends over a period of seconds. The transmitted signals are desirably low-frequency to reduce attenuation losses in the reflected waves. Unlike the previously mentioned devices which emit a short duration pulse and thereby provide a discrete echo at some readily determinable point in time, the chirp devices often vary the frequency of the transmitted signal in some pre-set manner so that a unique frequency in the reflected signal can be correlated as a function of time with that same frequency in the transmitted signal. A collection of received signals or "trace" can be mathematically manipulated to produce a subterranean map.
The transducer in subsea vibrator devices typically is an acoustic piston or plate in contact with the water and driven by a pneumatic or hydraulic actuator modulated at the desired frequency. An example of such a device is found in U.S. Pat. No. 4,211,301, to Mifsud. The patent does not discuss methods of continuously tuning the source for maximum output nor does it suggest the use of multiple ganged plates moving in unison to produce seismic waves.
There is a limit to the amount of energy that can be introduced into a subsea acoustic wave. That amount depends on, inter alia, the size of the transducer, the amplitude of oscillation, the depth of source placement, temperature and salinity of the water, and the frequency of the transmission. When this threshold amount of induced energy is exceeded, the source cavitates and produces gas bubbles rather than a clean acoustic signal. Nevertheless, the strength of the acoustic signal should be maximized to assure the strongest possible echo. The invention disclosed herein deals with apparatus suitable for maximizing the efficiency of such a marine seismic source by adjusting its output impedance so that a higher percentage of the input power is used to radiate acoustic energy.
Other marine seismic sources are known which provide for the prevention of cavitation. The disclosure in U.S. Pat. No. 3,691,516, to Graham et al, provides a description of an apparatus having a pair of acoustic pistons located at opposite ends of the device. The acoustic pistons are held outward from the center of the seismic source by a pair of variable volume chambers. The pressure within the variable volume chambers is repetitively varied downward and then returned to the initial value. This sharp reduction in pressure causes the pistons to move inward initiating the pulse. Hydraulic cylinders attached to the pistons via piston rods then tend to restore the pistons to their original extended positions. The acceleration rate of the pistons is controlled using a feedback loop so that the pistons produce the maximum possible acoustic output power as limited by the cavitation threshold. The acceleration rate is varied by pressure control of the fluid introduced into the aforementioned hydraulic cylinders. The frequency of the device is changed by a fulcrum and beam arrangement operating in conjunction with the piston-restoring hydraulic cylinders.
The Graham et al device produces a pulse which has the maximum power attainable for the physical size of the acoustic pistons in their particular surrounding fluid. The device desirably operates just below the cavitation threshold. The device does not vary its output impedance to maximize output at a particular available power input using the apparatus of the instant invention.
Other marine seismic sources which suggest tuning the source for maximum output are typified by: U.S. Pat. Nos. 3,349,367, to Wisotsky; 3,392,369, to Dickie et al: 4,030,063, to Wallen; and 4,142,171, to Pickens. Each of these patents, however, involves a single frequency source.